Mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogels

ABSTRACT

A method of making a mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel, including the steps of dispersing nanotubes in an aqueous media or other media to form a suspension, adding reactants and catalyst to the suspension to create a reaction mixture, curing the reaction mixture to form a wet gel, drying the wet gel to produce a dry gel, and pyrolyzing the dry gel to produce the mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel. The aerogel is mechanically robust, electrically conductive, and ultralow-density, and is made of a porous carbon material having 5 to 95% by weight carbon nanotubes and 5 to 95% carbon binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/147,694 filed Jan. 27, 2009entitled “mechanically robust, electrically conductive ultralow-densitycarbon nanotube-based aerogels,” the disclosure of which is herebyincorporated by reference in its entirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to aerogels and more particularly tomechanically robust, electrically conductive ultralow-density carbonnanotube-based aerogels.

2. State of Technology

Carbon Aerogels (CAs) are novel mesoporous materials with applicationssuch as electrode materials for super capacitors and rechargeablebatteries, adsorbents and advanced catalyst supports. Carbon nanotubes(CNTs) possess a number of intrinsic properties. CNTs have large aspectratios (100-1000).

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a method of making a mechanically robust,electrically conductive ultralow-density carbon nanotube-based aerogel,including the steps of dispersing nanotubes in an aqueous media or othermedia to form a suspension, adding reactants and catalyst to thesuspension to create a reaction mixture, curing the reaction mixture toform a wet gel, drying the wet gel to produce a dry gel, and pyrolyzingthe dry gel to produce the mechanically robust, electrically conductiveultralow-density carbon nanotube-based aerogel. The present inventionprovides a porous carbon material comprising 5 to 95% by weight carbonnanotubes and 5 to 95% carbon binder. In one embodiment the presentinvention provides a porous carbon material comprising 5 to 95% byweight carbon nanotubes and 5 to 95% organic binder porous organicmaterial that is pyrolyzed. Aerogels constructed according to thepresent invention have many uses. For example, aerogels constructedaccording to the present invention have use as electrode materials forsuper capacitors and rechargeable batteries, adsorbents and advancedcatalyst supports.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIGS. 1A-1D are SEM images of nanotube-based aerogels with (A) 4 wt %,(B) 20 wt %, (C) 30 wt %, and (D) 55 wt % CNTs.

FIGS. 2A and 2B show plots of (FIG. 2A) monolith density and CNTconcentration and (2B) volume shrinkage as a function of CNT loading.

FIG. 3 shows plots of the nanotube loading dependencies of the monolithdensity for foams.

FIG. 4 illustrates the dependence of Young's modulus on the monolithdensity for carbon aerogels.

FIG. 5 compares elastic properties of CNT-loaded nanofoams with those ofsome other low-density nanoporous systems.

FIG. 6 shows SEM images of the composites.

FIG. 7 plots the pore size distribution of the SWNT-CA, TiO₂/SWNT-CA,and pristine TiO₂ aerogel.

FIGS. 8A-8B illustrates one embodiment of a method of making amechanically robust, electrically conductive ultralow-density carbonnanotube-based aerogel.

FIGS. 9A-9D are electron microscope images of the aerogel.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

DEFINITION OF TERMS

Various terms used in this patent application are defined below.

-   -   CA=Carbon Aerogel    -   CAT=Carbon Nanotubes    -   CA-CNT=Carbon Aerogel & Carbon Nanotube Compsite    -   SWNT=Single-Walled Carbon Nanotubes    -   DWNT=Double-Walled Carbon Nanotubes    -   SDBS=Sodium Dodecylbenzene Sulfonate    -   MESOPORPOUS=Pore Dia. 2 & 5 mm    -   PVA=Polyvinyl Alcohol    -   CVD=Chemical Vapor Deposition    -   TEM=Transmission Electron Microscopy    -   SEM=Scanning Electron Microscopy    -   R/C=Resorcinol to Catalyst Ratios    -   RF=Resorcinol and Formaldehyde Solids    -   BET=Brunauer-Emmett-Teller    -   Mechanically Robust=Can withstand strains greater than 10%        before fracture    -   Electrically Conductive=Exhibits an electrical conductivity of        10 S/m or greater    -   Ultralow-Density=Exhibits densities less than 50 mg/cc    -   Carbon Nanotube-Based Aerogel=Porous carbon material consisting        of 5 to 95% carbon nanotubes by weight

The present invention provides a method of making a mechanically robust,electrically conductive ultralow-density carbon nanotube-based aerogel,The method includes the steps of dispersing nanotubes in an aqueousmedia or other media to form a suspension, adding reactants and catalystto the suspension to create a reaction mixture, curing the reactionmixture to form a wet gel, drying the wet gel to produce a dry gel, andpyrolyzing the dry gel to produce the mechanically robust, electricallyconductive ultralow-density carbon nanotube-based aerogel. The presentinvention also provides an aerogel that includes a porous carbonmaterial comprising 5 to 95% by weight carbon nanotubes and 5 to 95%carbon binder.

Applicants have developed a method to fabricate mechanically robust,electrically conductive low-density carbon nanofoams with macroscopicdimensions. The nanofoams are prepared by the sol-gel polymerization ofresorcinol with formaldehyde in an aqueous suspension containing adispersion of highly purified single-walled carbon nanotubes. Subsequentdrying and pyrolysis result in nanoporous solids consisting of a randomnetwork of carbon nanotube bundles decorated and crosslinked bygraphitic carbon nanoparticles. Such nanotube-based foams exhibitelectrical conductivities and elastic properties significantly improvedcompared to those of foams without nanotubes and elastic behavior up tocompressive strains as large as −v80%. They are the stiffest low-densitynanoporous solids reported and could find use in many energy-relatedapplications.

Applicants determined that double-walled CNTs can be incorporated into acarbon aerogel matrix at concentrations up to −8 wt % by starting withsurfactant-stabilized nanotube dispersions. The resultant compositesdisplay isotropic properties with enhanced electrical conductivities andelastic moduli compared to those of carbon aerogels without CNTs.

Applicants demonstrate how carbon aerogel nanoparticles can serve tocrosslink CNTs resulting in a new class of ultralow-density nanoporousmonoliths of macroscopic dimensions with unprecedented properties.Compared to Applicants' previous attempts of foam synthesis based onsurfactant-stabilized dispersions of double-walled CNTs, Applicants usepurified single-walled CNTs dispersed directly in water without the aidof surfactants. The resultant foam consists of a random network ofsingle-walled CNT bundles decorated and crosslinked by graphiticnanoparticles. We demonstrate nanofoams with CNT concentrations over 60wt % and monolith densities as low as −10 mg cm⁻³. These nanoporoussolids simultaneously exhibit remarkable mechanical stiffness, verylarge elastic strains, and high electrical conductivity even atdensities approaching −10 mg cm⁻³. The foams are stable at temperaturesover 1000° C. and have shown to be unaltered by exposure to extremelylow temperatures (−15 K) during immersion into cryogenic hydrogen.Hence, in addition to use in applications such as catalyst supports,sensors, adsorbents, and energy storage, these ultralight, robustnanofoams could be used as scaffolds for novel composites. As aconductive network is already established, it could be impregnatedthrough the wicking process with a matrix of choice, ranging from moltenmetals to polymer melts to ceramic pastes to cryogenic deuterium-tritiumliquids and crystals.

EXAMPLES

Sample Preparation.

A DWNT-CA composite was prepared using traditional organic sol-gelchemistry. Purified DWNTs (Carbon Nanotechnologies, Inc.) were suspendedin an aqueous surfactant solution containing SDBS and thoroughlydispersed using a Bronwill Biosonik IV tip sonicator operating at 25% ofmaximum power at high frequency. To determine the optimal conditions forDWNT dispersion, a range of sonication times (1 to 4 hrs) andSDBS-to-DWNT ratios (10:1, 5:1 and 2.5:1) were evaluated. Once the DWNTwere dispersed, resorcinol (1.235 g, 11.2 mmol), formaldehyde (1.791 g,22.1 mmol) and sodium carbonate catalyst (5.95 mg, 0.056 mmol) wereadded to the reaction solution. The resorcinol to catalyst ratio (R/C)employed for the synthesis of the composites was ˜200. The sol-gelmixture was then transferred to glass molds, sealed and cured in an ovenat 85° C. for 72 h. The resulting gels were then removed from the moldsand washed with acetone for 72 h to remove all the water from the poresof the gel network. The wet gels were subsequently dried withsupercritical CO2 and pyrolyzed at 1050° C. under a N2 atmosphere for 3h. The composite materials were isolated as black cylindrical monoliths.Carbon aerogel composites with DWNT loadings ranging from 0 to 8 wt % (0to 1.3 vol %) were prepared by this method. For comparison purposes,pristine CAs as well as SDBS-loaded CAs were also prepared using themethod described above, except without the addition of the DWNT.

Characterization. Bulk densities of the DWNT-CA composites weredetermined from the physical dimensions and mass of each sample. Thevolume percent of DWNT in each sample was calculated from the initialmass of DWNTs added, assuming a CNT density of 1.3 g/cm3, and the finalvolume of the aerogel. Scanning electron microscopy (SEM)characterization was performed on a JEOL 7401-F. SEM sample preparationincluded sputtering a few nanometer layer of Au on the aerogel sample.Imaging was done at 5-10 keV (20 mA) in SEI mode with a working distanceof 2-8 mm. Electrical conductivity was measured using the four-probemethod similar to previous studies. Metal electrodes were attached tothe ends of the cylindrical samples. The amount of current transmittedthrough the sample during measurement was 100 mA and the voltage dropalong the sample was measured over distances of 3 to 6 mm.

Materials.

All reagents were used without further purification. Resorcinol (99%)and formaldehyde (37% in water) were purchased from Aldrich Chemical Co.Sodium carbonate (anhydrous) was purchased from J.T. Baker Chemical Co.Sodium dodecylbenzene sulfonate (SDBS) was purchased from Fluka ChemicalCorp., Inc. Purified DWNTs were purchased from Carbon Nanotechnologies,Inc.

Results.

Based on our initial results, we found that tip sonication of DWNTs inaqueous solution of sodium dodecyl benzene sulfonate (SDBS) provided themost uniform dispersion of DWNTs in the CA matrix and, therefore, thisapproach was used to prepare the nanocomposites. In a typical synthesis,the DWNT were added to a solution of SDBS in water and dispersed using atip sonicator. Resorcinol, formaldehyde and the reaction catalyst werethen added to the solution and the reaction mixture was cured atelevated temperatures, during which time, black monolithic gels formed.These wet gels were then supercritically-dried and carbonized to affordthe DWNT-CA composites. Interestingly, during the solvent exchange stepprior to supercritical drying, the fluid washed from the pores of thewet gel was clear, indicating that the majority of DWNTs had beenincorporated into the aerogel structure. Using this approach, a seriesof CA composites with DWNT loading ranging from 0 to 8 wt % (0 to 1.3vol %) were prepared.

The microstructures of the DWNT-CA composites were evaluated usingscanning electron microscopy. As shown in FIGS. 1A-1D, the networkstructures of the CAs consist of interconnected networks of primarycarbon particles, as would be expected based on the sol-gel reactionformulation. This observation is important as it shows that theformation of the aerogel network is not negatively impacted by thepresence of either the surfactant or the DWNTs. These images also showthe distribution of DWNTs throughout the CA framework. Clearly, thecombination of SDBS surfactant and sonication was effective inmaintaining the dispersion of DWNT during the sol-gel polymerizationreaction. Based on the SEM images, the DWNTs are dispersed as bundleswith diameters of less than 10 nanometers, while the lengths of thesebundles are on the order of ˜1 micron. Not surprisingly, the compositesprepared with higher loading levels of DWNTs clearly show a higherpopulation of nanotubes in the SEM images.

FIGS. 1A and 1B show SEM images revealing that nanofoams with CNTloading below - - -20 wt % have the morphology of pristine carbonaerogels (i.e., conventional carbon aerogels without nanotubes),consisting of a randomly interconnected network of carbon nanoparticles.Hence, the process of gelation is not negatively impacted by thepresence of <20 wt % of CNTs in the matrix. FIGS. 1A and 1B also showthe uniform distribution of nanotubes throughout the aerogel framework,demonstrating that the acid fimetionalization of CNTs duringpurification and sonication were effective in maintaining the dispersionof CNTs during the sol-gel polymerization reaction. Nanotubes aredispersed as randomly-oriented bundles with diameters of <10 nm andlengths of −500-1000 nm.

FIGS. 1C and 1D show that, for nanotube loading >20 wt %, the foammorphology changes from a network of interconnected nanoparticles(typical of pristine carbon aerogels) to a network of randomlyinterconnected filament-like struts. The diameters of these filamentsrange from <5 nm to almost 40 nm. Transmission electron microscopyimages (FIG. 2) further show that, at least at the surface, thesestructures do not appear to be purely CNTs or their bundles as nonanotube walls are visible.

FIGS. 2A and 2B are two plots. FIG. 2A is a plot showing monolithdensity and CNT concentration. FIG. 2B is a plot showing volumeshrinkage as a function of CNT loading. To determine the effect thatincorporation of DWNTs into the CA matrix has on the electricalproperties of these materials, the electrical conductivity of theDWNT-CA composites were determined using the four-point probe method. Asshown in FIGS. 2A and 2B, the electrical conductivity of each compositematerial is enhanced relative to their respective pristine CA reference.The electrical conductivity enhancement, enhanced, is given by

$\begin{matrix}{\sigma_{enhanced} = \frac{\sigma_{{DWNT} - {CA}} - \sigma_{CA}}{\sigma_{CA}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$where sDWNT-CA and sCA are the measured electrical conductivities of theDWNT-CA composite and the pristine CA, respectively. This relativeenhancement in electrical conductivity was chosen over absoluteelectrical conductivity so that samples of different densities could bedirectly compared. In general, the electrical conductivity of thenanocomposites increases as a function of DWNT concentration.

The observation of filament-like struts in FIGS. 1C, 1D, and 2 indicatesthat, for CNT loading above −20%, the majority of nucleation during thesol-gel reaction occurred on nanotube walls instead of in the sol. Thisis consistent with previous observations of nucleation on the walls of10-micron-diameter carbon fibers introduced into a sol-gel reaction.

FIG. 3 shows plots of the nanotube loading dependencies of the monolithdensity for foams prepared with different concentrations of resorcinoland formaldehyde (RF) solid content in the starting solution and volumeshrinkage for 4 wt % RF solid content for a resorcinol to catalyst (R/C)ratio of 50. The monoliths prepared with a R/C of 200 showed similartrends. FIG. 3 a reveals that the monolith density decreases withincreasing CNT loading for all RE solid contents. Formulations thatwould typically yield pristine carbon aerogels with densities of - --200-350 mg cm⁻³, with addition of CNTs to the reaction mixture, canachieve densities of −30-150 mg cm⁻³. Such a decrease in the monolithdensity is related to smaller volume shrinkage (FIG. 3) during bothsupercritical drying and pyrolysis. This dramatic effect of nanotubeloading on monolith shrinkage is clearly illustrated in the inset ofFIG. 3 b, showing a series of samples gelled in molds of the same sizebut shrunk to various degrees during drying and pyrolysis depending onnanotube loading.

FIG. 4 illustrates the dependence of Young's modulus on the monolithdensity for carbon aerogels with different nanotube loading and R/Cratios of 50 (closed symbols) and 200 (open symbols). Data from previousstudies of carbon, silica, and alumina aerogels, are also shown forcomparison. The inset shows a sequence of uniaxial compression of amonolith with a density of 30 mg cm⁻³ and a CNT loading of 55 wt %,illustrating a “superelastic” behavior with complete strain recoveryafter compression to strains as large as −76′/0.

The reduced shrinkage of CNT-based aerogels could be attributed to theirimproved mechanical properties with increasing CNT loading. FIG. 4 showsa double logarithmic plot of the dependence of Young's modulus on themonolith density for nanofoams with different CNT loading. It is seenfrom FIG. 4 that moduli for pristine aerogels correlate well with datareported by Pekala et al., (21) with aerogels prepared using a smallerR/C ratio of 50 being stiffer than those prepared with a larger R/Cratio. It is also seen from FIG. 4 that the elastic modulus E exhibits apower-law dependence on the material density, with an exponent in of−2,5. All points for carbon aerogels roughly fall on two lines, whichare extrapolations of the dependencies for pristine aerogels preparedwith R/C ratios of 50 and 200. Nanofoams with CNT loading of <16 wt %show no significant improvement of the elastic modulus over that forpristine carbon aerogels, while moduli of all foams with CNT loading >16wt % fall on the same line, independent of loading values and the R/Cratio. This observation is in agreement with the dramatic change in foammorphology, reduced shrinkage, and improved electrical conductivity (seebelow) for foams with large CNT loading.

FIG. 5 also compares elastic properties of CNT-loaded nanofoams withthose of some other low-density nanoporous systems, such as conventionalcarbon, silica, and alumina aerogels previously reported in theliterature. It illustrates unprecedented mechanical properties ofnanotube-based foams. Indeed, for a given density, the nanotube-basedaerogels are the stiffest. For example, for a density of 100 mg cm⁻³,foams with CNT loading above 16 wt. % are −12 and −3 times stiffer thanconventional silica and carbon aerogels, respectively. Nanotube-basedfoams are also −3 times stiffer than the “superstiff” alumina nanofoamswhose struts have the morphology of curled nanoleaflets. FIG. 4 shows asequence of images taken of an 30 mg cm⁻³ aerogel with a CNT loading of55 wt % before, during, and after uniaxial loading up to a maximumstrain of −76%. It shows a “superelastic” behavior with complete strainrecovery that we have observed for nanotube-based foams with densitiesbelow −50 mg cm⁻³.

FIG. 5 illustrates the dependence of electrical conductivity ofnanotube-based aerogels on the monolith density. Data are for foamsprepared with R/C ratios of 50 (closed symbols) and 200 (open symbols)and with nanotube loading of 0 and 30 wt %. The inset shows a dramaticincrease in the effective conductivity of foam strut (defined in thetext) with increasing nanotube loading above −16 wt %.

FIG. 5 shows the dependence of electrical conductivity, u, ofnanotube-based aerogels on the monolith density, p, for a set ofaerogels prepared with R/C ratios of 50 and 200 and with nanotubeloading of 0 and 30 wt %. Such a double-logarithmic plot reveals apower-law nature of the u(p) dependence: o p″, with an exponent n of−1.55 for both sets of foams with different CNT loading and independentof the R/C ratio. Previous observations for pristine aerogels by Lu etal. are consistent with results of FIG. 5.

Although the exponent n is independent of CNT loading, FIG. 5 clearlyshows that, the electrical conductivity is −3 times larger for aerogelswith 30 wt % CNTs than for pristine aerogels of the same density,indicating that the effective conductivity of aerogel struts, increaseswith increasing CNT loading. This effect is better illustrated in theinset in FIG. 5, showing that cs. dramatically increases for CNT loadingabove −16%, independent of the R/C ratio. Hence, conductivity data areconsistent with behavior of monolith shrinkage, elastic properties, andchanges to foam morphology discussed above. For a CNT loading of - - -60wt %, an - -5 time increase in conductivity is observed. Theclose-to-linear dependence of the effective conductivity of foam strutson CNT loading revealed by the inset in FIG. 5 is expected for strutsmade of nanotube bundles decorated and interconnected with graphiticcarbon nanoparticles (FIG. 1). In this case, strut conductivity isdominated by the resistivities of tube bundles and an array of graphiticcarbon aerogel particles connected in parallel.

To demonstrate how the SWNT-CAs could serve as conductive scaffolds tomake a wide range of conductive composites, SWNT-CAs were infiltratedwith insulating aerogels and polymers to form composites. The compositeswere made by simply immersing the scaffold in the precursor sol or resinunder vacuum prior to curing. After curing, supercritical extraction wasused to yield the aerogel composites. Table 1 gives the CNT content,density, BET surface area, and electrical conductivity of SWNT-CAscaffolds and various composites made with the SWNT-CA scaffolds. FIG. 6shows SEM images of the composites and FIG. 7 plots the pore sizedistribution of the SWNT-CA, TiO₂/SWNT-CA, and pristine TiO₂ aerogel.Table 1 shows that, in the case of the aerogel composites (TiO₂ andSiO₂), the high surface area and electrical conductivity of the SWNT-CAis not adversely affected by the infiltration of the insulatingmaterial. Though, based on the SEM images (FIGS. 6A-6B), the oxideaerogels appear to simply coat the SWNT-CA scaffold, the increasedsurface area suggests that the pore morphology of the oxide aerogelsdominate. This is confirmed in the TiO₂ case via the pore sizedistribution (FIG. 7). FIG. 7 shows that the pore size distribution ofthe TiO₂/SWNT-CA is much closer to that of pristine TiO₂ aerogel insteadof the SWNT-CA. Similar observations were made with the SiO₂/SWNT-CA.Thus, with the oxide aerogel composites, the high conductivity of theSWNT-CA is maintained exhibiting the pore morphology of the oxideaerogel.

TABLE 1 CNT, Density, Material vol % (wt %) g/cm³ S_(BET, m) ^(2/g)Sem⁻¹ SWNT-CA-1 0.5 (30)   0.030 184 0.77 SWNT-CA-2 1 (55) 0.028 1621.12 TiO₂/SWNT-CA-1 0.5 (8)   0.082 203 0.72 SiO₂/SWNT-CA-2 1 (16) 0.080652 1.00 Epoxy/SWNT-CA-2  1 (1.5) 1.120 NA 1.01

In the case of the polymer composite, Table 1 shows that the electricalconductivity of the SWNT-CA is maintained even in a fully denseinsulating matrix. Little to no change in the conductivity indicatesthat the conductive scaffold is intact. SEM images of the epoxy/SWNT-CA(FIGS. 6C-6D) confirm that the conductive network is intact and theSWNTs are homogenously distributed throughout the polymer matrix. Theconductivity of this material, to our knowledge, represents the highestreported conductivity (I Scm⁻¹) at this CNT loading level (1.5 wt % or Ivol %) for an epoxy composite. The properties of the aerogel and polymercomposites suggest that the SWNT-CA would serve as an excellentfoundation for the development of a wide range of conductive composites.

The largest improvements in electrical conductivity were observed inDWNT-CAs with 8 wt % (1.3 vol %) DWNTs, showing a twofold increase inconductivity. To verify that these enhancements were attributable to theincorporated DWNTs and not the SDBS surfactant, we also measured theelectrical conductivity of reference CA materials that were preparedwith SDBS and without the DWNTs. While the data for these materials showmodest improvements in conductivity relative to the pristine CA, theeffect is small relative to the overall enhancements seen in the DWNT-CAcomposites. Therefore, these improvements can be attributed to theincorporation of DWNTs into the CA framework.

Referring again to the drawings and in particular to FIGS. 8A and 8B,one embodiment of a method of making a mechanically robust, electricallyconductive ultralow-density carbon nanotube-based aerogel isillustrated. The method is designated generally by the reference numeral800. The method 800 includes steps 802-816.

Step 802—Obtain: Resorcinol, form-aldehyde, sodium carbonate,sodium-dodecylbenzene sulfunate (SDBS) and purified double-wallednanotubes (DWNT).

Step 804—Purified DWNTS suspended in aqueous solution containing SDBS.

Step 806—Dispersal of DWNTS in aqueous surfactant solution containingSDBS using sonication.

Step 808—Resorcinol, formaldehyde and sodium carbonate catalyst added toreaction solution.

Step 810—Sol-gel mixture transferred to glass mods, sealed and cured inoven at 85° C. for 72 hours.

Step 812—Resulting gel removed from mold and washed with acetone for 72hours to remove all water from pores of gel network.

Step 814—Wet gel dried with supercritical CO2 and pyrolyzed at 1050° C.under N2 atmosphere for 3 hours.

Step 816—Resulting composite material (CA-DWNT) isolated as blackcylindrical monoliths.

Referring to the FIGS. 9A-9D electron microscope images show oneembodiment of an aerogel of the present invention. FIGS. 9A and 9B areelectron microscope images at 200 nm. FIG. 9C is an electron microscopeimages at 100 nm. FIG. 9D is an electron microscope images at 20 nm. Theligaments are the nanotubes and the coating on the nanotubes are thecarbon nanoparticles that serve as the binder holding the nanotubestogether.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

The invention claimed is
 1. A method of making a mechanically robust,electrically conductive ultralow-density carbon nanotube-based aerogel,comprising the steps of: dispersing carbon nanotubes in an aqueous mediaor other media to form a suspension, adding reactants and catalyst tosaid suspension to create a reaction mixture, wherein the reactionmixture is substantially free of any surfactant for said carbonnanotubes, curing said reaction mixture to form a wet gel, drying saidwet gel at to produce a dry gel, and pyrolyzing said dry gel atemperature above 600° C. under an inert atmosphere to produce themechanically robust, electrically conductive ultralow-density carbonnanotube-based aerogel, wherein the mechanically robust, electricallyconductive ultralow-density carbon nanotube-based aerogel comprises anetwork of carbon nanotubes crosslinked by carbon nanoparticles.
 2. Themethod of claim 1, wherein said step of dispersing carbon nanotubes inan aqueous media or other media to produce a suspension comprisesdispersing single-walled carbon nanotubes in an aqueous media or othermedia.
 3. The method of claim 1, wherein said step of dispersing carbonnanotubes in an aqueous media or other media to produce a suspensioncomprises dispersing double-walled carbon nanotubes in an aqueous mediaor other media.
 4. The method of claim 1, wherein said step ofdispersing carbon nanotubes in an aqueous media or other media toproduce a suspension includes sonicating said aqueous media or othermedia containing nanotubes.
 5. The method of claim 1, wherein said stepof adding reactants and catalyst to said suspension to create a reactionmixture includes adding resorcinol to said suspension to create areaction mixture.
 6. The method of claim 1, wherein said step of addingreactants and catalyst to said suspension to create a reaction mixtureincludes adding formaldehyde to said suspension to create a reactionmixture.
 7. The method of claim 1, wherein said step of adding reactantsand catalyst to said suspension to create a reaction mixture includesadding sodium carbonate to said suspension to create a reaction mixture.8. The method of claim 1, wherein said step of curing said reactionmixture comprises curing said reaction mixture at a temperature above25° C. for an extended period of time.
 9. The method of claim 1, whereinsaid step of drying said wet gel comprises washing said wet gel bysolvent exchange.
 10. The method of claim 1, wherein said step of dryingsaid wet gel comprises washing said wet gel with acetone.
 11. The methodof claim 1, wherein said step of drying said wet gel comprised dryingsaid wet gel with a supercritical fluid.
 12. The method of claim 1,wherein said step of drying said wet gel comprised drying said wet gelwith CO₂.
 13. The method of claim 1, wherein the carbon nanotube-basedaerogel comprises at least 20 wt. % of carbon nanotubes.
 14. The methodof claim 1, wherein the carbon nanotube-based aerogel comprises at least55 wt. % of carbon nanotubes.
 15. The method of claim 1, wherein thereaction mixture is free of any surfactant for said carbon nanotubes,and wherein the carbon nanotube-based aerogel comprises at least 30 wt.% of carbon nanotubes.
 16. The method of claim 1, wherein the carbonnanotube-based aerogel comprises a network of interconnectedfilament-like struts.
 17. The method of claim 1, further comprisingimmersing said carbon nanotube-based carbon aerogel in a polymer resinto produce a carbon nanotube-based carbon aerogel infiltrated with saidpolymer resin, and curing said carbon nanotube-based carbon aerogelinfiltrated with said polymer resin to produce a composite of saidcarbon nanotube-based carbon aerogel and said polymer.
 18. The method ofclaim 1, further comprising immersing said carbon nanotube-based carbonaerogel in a precursor sol of metal oxide to produce a carbonnanotube-based carbon aerogel infiltrated with said precursor sol, andcuring said carbon nanotube-based carbon aerogel infiltrated with saidprecursor sol to produce a composite of said carbon nanotube-basedcarbon aerogel and said metal oxide.
 19. The method of claim 1, whereinthe reaction mixture comprises resorcinol and formaldehyde as reactantsand is substantially free of any surfactants for the carbon nanotubes,wherein said step of curing said reaction mixture comprises curing saidreaction mixture at a temperature above 25° C. for an extended period oftime, wherein said step of drying said wet gel comprises washing saidwet gel by solvent exchange and drying said wet gel with supercriticalCO₂, and wherein said step of pyrolyzing said dry gel comprisingpyrolyzing said dry gel at a temperature above 600° C. under an inertatmosphere.
 20. A mechanically robust, electrically conductiveultralow-density porous carbon nanotube-based aerogel obtained by themethod of claim
 1. 21. The aerogel of claim 20, comprising 5 to 95% byweight of carbon nanotubes and 5 to 95% by weight of carbon binder. 22.The aerogel of claim 20, wherein the carbon nanotubes are single-walledcarbon nanotubes.
 23. The aerogel of claim 20, wherein the aerogelincludes ligaments and a coating on said ligaments and wherein saidligaments are said carbon nanotubes and wherein said coating on saidligaments are carbon nanoparticles that serve as a binder holding saidcarbon nanotubes together.